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CAIE 9618 · Paper 1 · Topic 1.5.1

Operating System
Functions

Process Management · Memory Management · File System · I/O · Security

CSZone Cambridge International AS & A Level Computer Science 9618
What is an Operating System?

The Software Layer Between Hardware and User

An Operating System (OS) is system software that manages computer hardware and software resources, providing an environment in which application programs can run. It acts as an intermediary between the user, applications, and hardware.
🖥️
Process Mgmt
🧠
Memory Mgmt
📁
File System
🔌
I/O Management
🔒
Security
🌐
Networking
Process Management

Scheduling & Multitasking

The OS manages all running processes (programs in execution). In a single-processor system, only one process runs at a time, but the OS switches rapidly between them — creating the illusion of multitasking.
Scheduler — decides which process runs next and for how long (time-slice)
Round Robin — each process gets an equal time-slice in turn; fair, prevents starvation
Priority Scheduling — higher priority processes run first; OS processes typically have highest priority
Process states: Running → Waiting → Ready → Running (cycle managed by OS)
Deadlock: two processes each waiting for a resource held by the other — OS must detect and resolve
Memory Management

Allocating and Protecting RAM

Allocation — assigns RAM space to each running process
Protection — prevents processes from accessing each other's memory (isolation)
Virtual memory — manages paging between RAM and disk (see 1.3.6)
Memory compaction — reorganises memory to reduce fragmentation
MEMORY MANAGEMENT UNIT (MMU)
Hardware component that translates virtual addresses to physical addresses. Works with the OS page table to provide each process its own virtual address space, ensuring isolation and security.
File System Management

Organising and Accessing Files

File naming, creation, deletion — provides user-visible naming; tracks where files are physically stored on disk
Directory structure — hierarchical tree of folders; full path uniquely identifies every file
Access permissions — read/write/execute rights per user or group; OS enforces these
File allocation table / index — OS maintains a map of which sectors belong to which file
Metadata — file size, creation/modification date, owner, type, permissions stored by OS
I/O Management & Device Drivers

Controlling Hardware Devices

DEVICE DRIVERS
Software that translates OS commands into device-specific instructions. Each hardware device needs its own driver (e.g. printer, graphics card, NIC). OS provides a standard interface — applications don't need to know hardware specifics.
I/O BUFFERING & SPOOLING
Buffer: temporary storage for data in transit (e.g. keyboard buffer). Spooling: queues print jobs for a slow printer — allows user to continue working while printer catches up. Managed transparently by OS.
Exam Practice

Cambridge-style questions

Question 1
Describe three functions of an operating system, giving an example of each in context.
6 marks
2 marks
Process management: OS uses a scheduler to decide which process gets CPU time. e.g. Round Robin allocates equal time-slices to each running application.
2 marks
Memory management: OS allocates separate areas of RAM to each process and prevents them accessing each other's memory. e.g. OS ensures a web browser cannot read another application's data.
2 marks
File system management: OS manages creation, deletion, and access to files. e.g. OS enforces read-only permission on a system file so user programs cannot accidentally overwrite it.
Common Mistakes

Don't lose easy marks

1
Saying the OS runs programs — the OS provides the environment in which programs run. The CPU executes program instructions; the OS manages resources around them. The OS itself is just software.
2
Listing "providing a user interface" as a primary OS function — while most OS provide GUIs or CLIs, this is a layer on top of the OS (shell/desktop environment). Core OS functions are process management, memory management, file system, and I/O management.
3
Confusing process and program — a program is a static set of instructions stored on disk. A process is a program in execution (actively using CPU and memory). Multiple processes can run from the same program simultaneously.
Topic Summary — 1.5.1

What You Need to Know

PROCESS MANAGEMENT
Scheduling (Round Robin, Priority). States: Running/Waiting/Ready. Deadlock detection. Multitasking illusion.
MEMORY MANAGEMENT
Allocate RAM per process. Memory protection (isolation). Virtual memory. MMU translates addresses.
FILE SYSTEM
Naming, creation, deletion. Hierarchical directories. Access permissions. Metadata management.
I/O MANAGEMENT
Device drivers: translate OS → hardware commands. Buffers: temporary data storage. Spooling: print queues.
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1.5.2
System & Utility Software
OS Types · Utilities · Backup · Antivirus · Defrag
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